Preparation and Characterization of Nanohybrids Made of Graphene Oxide as Super Adsorbents

. Abstract Adsorption is considered one of the best methods for the removal of heavy metal ions from an aqueous solution. However, the synthesis of adsorbents with desired selectivity and performance remains a key challenge in the battle of water decontamination. Recently, carbon-based and metal-oxide based nanomaterials have emerged as promising candidates for the adsorption of heavy metals due to their high specific surface area, high aspect ratio, and concentrated pore size distribution. Here, in this work five adsorbents ie. Graphene Oxide (GO), Magnetic Graphene Oxide (MGO), Titanium Dioxide (TiO 2 ), and their composites GO-TiO 2 and MGO-TiO 2 were synthesized. The prepared samples were characterized via high-resolution imaging, BET-N 2 adsorption-desorption analysis, and spectroscopic techniques. TEM results revealed the nanoscale structures of the synthesized nanomaterials. The approximate sizes of MGO and TiO 2 nanoparticles found under TEM studies were about 24.58 and 35.51 nm respectively. The presence of desired functional groups was very well deciphered by FT-IR spectroscopy. Results of N 2 adsorption-desorption studies revealed that the prepared GO was macro-porous while all other samples were mesoporous. MGO was found to have the highest BET surface area of about 108.375 m 2 /g. These results indicate that the prepared nanomaterials may serve the purpose of effectively adsorbing the heavy metal ions from an aqueous solution.


Introduction
The discharge of heavy metals into aquatic ecosystems has raised global concerns over the past few decades. These contaminants are introduced into water bodies through effluents from various industries, including paper and pulp, petrochemicals, automobiles, and battery manufacturing. While several methods, such as precipitation, ion exchange, reverse osmosis, and membrane filtration, have been employed to remove heavy metals. Most of these methods have drawbacks, such as high cost, low efficiency, and sludge generation . Additionally, they fail to meet the demand for water resources, particularly for large volumes. Adsorption, on the other hand, is a simple, economical, and adaptable method in terms of unit design. Natural and synthetic adsorbents, such as clay, activated carbon, mesoporous silica, and resin, have been used to treat heavy metal-contaminated water. However, separating and regenerating these adsorbents from wastewater poses a significant challenge. Therefore, there is a need to develop novel adsorbents with high adsorption capacity and quick separation from large volumes of water (Almomani et al., 2019). Recently, nanomaterials, particularly carbon-based and metal oxide-based ones, have emerged as promising adsorbents for heavy metal ions due to their high specific surface area, surface-to-volume ratio, and concentrated pore size distribution (Khan et al., 2013;Qu et al., 2015). Among these, graphene oxide (GO), a carbon-based nanomaterial, has received widespread attention (Yan et al., 2014, Majumder P, Gangopadhyay R. 2022. GO consists of a hexagonal network of covalently bonded carbon atoms with oxygencontaining functional groups, such as hydroxyl, epoxy, lactone, quinone, phenol, anhydride, carbonyl, ether, and carboxyl groups, attached to various sites, which facilitate the binding of positively charged metal ions to its surface (Guerrero-Fajardo et al., 2020; Zhao et al., 2011). However, GO's hydrophilicity and tendency to agglomerate during its application and storage make it challenging to separate, even after saturation adsorption Sun et al., 2015). To address these issues, GO can be magnetized, for example, with iron oxides. Nano-sized iron oxides exhibit superparamagnetism, low toxicity, chemical inertness, and the ability to immobilize various adsorbents on their surface (Jawed et al., 2020). The magnetized GO can be easily separated using an external magnetic field (Lingamdinne et al., 2019). Another category of nanomaterials that has shown favourable adsorption towards heavy metals is nano-sized metal oxides (Hua et al., 2012). Functionalizing GO with metal oxides largely increases the electronegative charge on its surface thereby improving metal removal efficiency (Jawed et al., 2020). The inclusion of active materials such as manganese dioxide (Xiang et al., 2018), iron oxide (Tian et al., 2017), and titanium dioxide (Liu et al., 2016) have sparked a profound interest in the field of adsorption as the metallic compounds could improve the adsorption performance by providing more active sites (Lai et al., 2020). Titanium dioxide nanoparticles are widely used as an adsorbent for heavy metal ions due to their low cost, stability, and non-toxicity towards human beings and the environment (Seidlerová et al., 2016). Here in this work the synthesis of adsorbents such as Graphene Oxide (GO), Magnetic Graphene Oxide (MGO), Titanium Dioxide (TiO 2 ), and their composites GO-TiO 2 , and MGO-TiO 2 has been reported. GO-TiO 2 is synthesized keeping in view the favourable properties of both GO and TiO 2 . MGO-TiO 2 nanocomposites are synthesized aiming at integrating the advantages of all the components -GO, TiO 2 along with magnetic properties of iron oxide for the added advantage of easier regeneration of adsorbents and improve the overall adsorption efficiency.

Material and Methods
Graphite fine powder extra pure of size 10 ), isopropyl alcohol (IPA), titanium isopropoxide (TTIP), isopropanol, HCl, and anhydrous ethanol were procured from Molychem, and were of analytical grade and used without any further purification.

Synthesis of Adsorbents
GO was prepared by the modified Hummer's method. Graphite powder was added into a mixture of concentrated H 2 SO 4 and H 3 PO 4 in the ratio of 9:1 under continuous stirring followed by the addition of KMnO4 to bring about the oxidation. The mixture was stirred continuously till the color changed to purple. Then the mixture was cooled down to room temperature and an ice water mixture and H 2 O 2 were added to stop the reaction. A yellow-colored solution was obtained which was then sonicated to bring about the complete exfoliation of graphite oxide. Then the solution was centrifuged once with HCl and many times with distilled water until a neutral pH was obtained. Finally, solid GO was obtained by vacuum drying in an oven and grinding using a pestle and mortar (Kaur and Jeet 2017).
The observations recorded during the synthesis of GO are given in figure 1.

Fig. 2: Observations recorded during the synthesis of MGO
MGO was prepared by the co-precipitation of iron oxide nanoparticles on the surface of GO. A dispersion of GO was prepared by adding a small amount of prepared GO in 100 ml of double-distilled water and sonicating it for an hour. Then, 10.7 g of ferric ammonium sulfate and 5.8 g of ferrous ammonium sulfate were added to 100 ml of doubledistilled water followed by the rapid addition of 10 ml of aqueous ammonia. The solution was then kept under stirring and GO suspension was slowly added to this solution after that the stirring was continued for about 45 minutes at 85 0C and then the solution was kept undisturbed overnight. Finally, MGO was separated with a magnet and washed thrice with distilled water and anhydrous ethanol respectively. Subsequently, MGO was kept in an oven at 70 0C for drying (Deng et al., 2013, Yi et al. 2021). The observations recorded during the synthesis of MGO are given in figure 2.
Titanium Dioxide was prepared by a wet chemical method. 10 ml of TTIP was added into a solution containing ethanol and double distilled water in the ratio of 7:1 under constant stirring. The stirring was further continued at room temperature until a thick paste with lots of nanoparticles was obtained, which were then separated by centrifugation with distilled water. The nanoparticles were then calcined and stored in an airtight container (Tamilselvi et al., 2016, MironyukI et al. 2020. The observations recorded during the synthesis of TiO 2 are given in figure 3.

Characterization
The surface area of the prepared nanomaterials was examined by BET N 2 adsorption-desorption analysis using Micromeritics ASAP 2020 volumetric adsorption analyzer installed at Emerging Life Sciences, Guru Nanak Dev University, Amritsar, Punjab, India. Before each analysis, each sample was degassed for 4-6 hours at 2000C. For the determination of the elemental composition and functional groups present the prepared samples were analyzed using Perkin Elmer Fourier Transform Infrared Spectrometer installed at Central Instrumentation Facility, Lovely Professional University, Phagwara, Punjab, India. The prepared samples were viewed under Transmission Electron Microscopy (TEM) installed at the Electron Microscopy and Nanoscience Lab, Punjab Agricultural University, Ludhiana, Punjab, India, to study their structural and morphological properties.

Results and Discussions BET N 2 Adsorption-Desorption Analysis
Magnetic Graphene Oxide was found to have the largest surface area of 108.3750 m 2 /g. The magnetization of GO appeared to increase its surface area, as evidenced by the larger surface area of MGO compared to GO. The surface areas of both the nanocomposites ie. GO-TiO 2 and MGO-TiO 2 were less than that of MGO which showed that the substitution of graphene oxide with titanium dioxide decrease its surface area. Due to the agglomeration of individual nanoparticles, the surface area of TiO 2 nanoparticles was only 4.42 m 2 /g. The extremely high calcination temperature may have contributed to the high average particle size of TiO 2 particles. As a result, the individual TiO 2 nanoparticles were agglomerated which was also evident from the TEM micrograph of TiO 2 nanoparticles. The pore diameters of the synthesized samples confirmed the presence of mesopores as per the IUPAC classification except for GO which was found to be macroporous (Thommes et al., 2015). The BET surface areas, pore sizes, and pore volume distribution of the synthesized samples are given in Table 1. Transmission Electron Microscopy Figure 6 shows the TEM micrographs of the prepared samples. The TEM micrograph of GO captured at 500 nanometers ( Figure 6a) reveals a layered-wrinkled structure with many folds on it, which may be attributed to various oxygencontaining functional groups (

Conclusion
The morphologies and size of the prepared nanomaterials were determined using transmission electron microscopy. TEM micrograph of GO depicted the formation of the layered structure of GO with many folds which might be attributed to the presence of oxygen-containing functionalities, TiO 2 micrograph depicted the formation of large agglomerates of TiO 2 particles. MGO-TiO 2 revealed the deposition of iron oxide nanoparticles and TiO 2 nanoparticles on the surface of GO. From the TEM micrographs the approximate sizes of MGO and TiO 2 nanoparticles were found to be about 24.58 and 35.51 nm respectively. FT-IR spectra of the synthesized nanomaterials depict the presence of required functional groups on their respective surfaces. N2 adsorption-desorption studies exhibited that MGO was having the highest surface area (108.3750 m2/g), and the prepared GO was macroporous while all other samples were mesoporous. Thus, the successful preparation of the adsorbents was confirmed.
Transmission electron microscopy was used to measure the produced nanomaterials' sizes and morphologies. A TiO2 micrograph showed the formation of huge agglomerates of TiO2 particles, and a TEM image of GO showed the development of a layered structure with many folds that may be explained by the presence of oxygen-containing functionalities. TiO2 and iron oxide nanoparticles were found to have been deposited on the surface of GO by MGO-TiO2. The approximate diameters of MGO and TiO2 nanoparticles were determined from the TEM micrographs to be around 24.58 and 35.51 nm, respectively. The presence of necessary functional groups on the surfaces of the synthesized nanomaterials is shown in their FT-IR spectra. Studies on the N2 adsorption-desorption of several materials showed that MGO had the greatest surface area (108.3750 m2/g), and that the produced GO was macroporous whereas all other samples were mesoporous. Thus, the adsorbents' successful preparation was verified. The results of the study provide information about the functional groups present in nanomaterials, which further aids in determining the heavy metal contamination that can be eliminated from aqueous medium, making the study highly beneficial for the synthesis of real-time filters. Moreover, surface area-related information offers binding site-related information.
Better adsorption capability is demonstrated by higher surface area. The usage of GO as an adsorbent indicated an increase in the strength of the filtration material.